Virt: change instance_type to flavor

The commit Ieebfcbc2478ba8d7036c1af30efbbf640aa7df73 made
use of instance_type. This should have been flavor.

This is related to blueprint flavor-instance-type-dedup.

Change-Id: Ib7cb7a0926050e3d0873328f84e8554884a48468
Merge "Virt: change instance_type to flavor"